It could be any of a couple dozen things. I'd have to supply lots more detail. I'd start by looking at file permissons. One common reason for not being able to read a file is that you don't have read permission.

If it works in one way you know it's not a network connectivity problem so you know it's something else.

When you go to System Preferences -> Sharing -> File Sharing should be checked. On 10.5, you can specify certain folders to share, and permissions per folder. An administrator's credentials will work to view the entire drive. Make sure this is checked on both machines. Also, there is an options button. Make sure AFP is checked (if both machines are macs), and/or make sure SMB is checked (if sharing to a Windows machine).

What are the specs of the two machines? Both Macs? Are they both 10.5?

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